"When the risk [of a significant negative reaction to its drug] became apparent, Wyeth had a duty to provide a warning that adequately described that risk," wrote Justice John Paul Stevens for the majority opinion.

The decision puts drug companies on notice that it is their responsibility, not that of the Food and Drug Administration, to ensure that its products feature clear warning labels.

Diana Levine, a professional guitarist, sued drug maker Wyeth, claiming that the company had given insufficient warning of the possible severe effects of one of its antinausea drugs called Phenergan. Her right arm was amputated after the drug was improperly injected into her arm, causing the onset of gangrene.

Investigation revealed that Wyeth was aware of 20 other individuals who had limbs amputated following the improper injection of Phenergan.

A Vermont jury awarded Ms. Levine $6.7 million in compensatory and punitive damages. The award was upheld by the Vermont Supreme Court.

The drug company appealed to the US Supreme Court, arguing that it would be difficult for pharmaceutical companies to operate under both federal regulations and assorted state laws.

The majority justices disagreed. "It is not impossible for Wyeth to comply with its state and federal law obligations," Justice Stevens wrote. Federal law allows drug companies to unilaterally strengthen warnings, he said in the opinion.

In a dissent, Justices Samuel Alito, Antonin Scalia, and Chief Justice John Roberts, said the Vermont labeling lawsuit should be preempted by federal regulations and prior labeling decisions made by the FDA.

"The FDA has long known about the risks associated with [injected use of Phenergan]," Justice Alito wrote. "Whether wisely or not, the FDA has concluded – over the course of extensive 54-year-long regulatory proceedings – that the drug is 'safe' and 'effective' when used in accordance with its FDA-mandated labeling."

Alito said the drug label includes adequate warnings, but that the medical technician who injected the drug into Levine's arm ignored six separate warning instructions. "[Levine] would be hard pressed to prove that a seventh would have made a difference," he said.

The case, Wyeth v. Levine (06-1249), is part of an emerging trend at the high court examining the scope of federal preemption of state laws. In December, the court sided with a group of smokers in Maine who filed a fraud lawsuit against Philip Morris for suggesting that their "light" and "low tar" cigarettes are a healthier alternative to regular cigarettes. The tobacco company had urged the high court to throw the suit out as preempted by federal law.

In February 2008, a different lineup of justices ruled that individuals injured by a defective medical device could not sue the manufacturer if the device received pre-market approval by the Food and Drug Administration.

The 8-1 ruling in the medical device case effectively insulates many corporations from potentially expensive product liability litigation. It has been cited as evidence of an emerging pro-business tilt at the high court under Chief Justice John Roberts.

In contrast, the Levine and Maine smoker decisions handed down this term suggest a tilt toward consumer protection and away from business interests.

In the Levine case, the question was whether state-based consumer lawsuits filed against FDA-regulated drug companies were preempted by federal law.

After losing her arm, Levine sued the technician who incorrectly injected the drug into her arm. She also sued the clinic where the injection took place. That case was settled out of court for about $700,000.

Levine's lawyers also filed the lawsuit against Wyeth, claiming the company's warnings on the drug label were not explicit enough to satisfy Vermont failure-to-warn laws protecting consumers.

In his majority opinion, Stevens rejects claims that Congress intended the FDA to exert exclusive control at the expense of the states.

"If Congress thought state law suits posed an obstacle to its objective, it surely would have enacted an express preemption provision at some point during the [federal drug laws's] 70-year history," Stevens wrote. "But despite its 1976 enactment of an express preemption provision for medical devices, Congress has not enacted such a provision for prescription drugs."

Stevens said that rather than thwarting federal goals, state-based lawsuits compliment FDA regulations and provide an additional layer of consumer protection. "The FDA has limited resources to monitor the 11,000 drugs on the market, and manufacturers have superior access to information about their drugs, especially in the postmarketing phase as new risks emerge," he said.

"State tort suits uncover unknown drug hazards and provide incentives for drug manufacturers to disclose safety risks promptly," he said. "They also serve a distinct compensatory function that may motivate injured persons to come forward with information. Failure-to-warn actions, in particular, lend force to the [federal drug law's] premise that manufacturers, not the FDA, bear primary responsibility for their drug labeling at all times."

In his dissent, Alito said juries are ill-equipped to perform the FDA's cost-benefit balancing. The courtroom focus on victims can obscure the wider calculation of probabilities and potentialities of a drug's usefulness for others.

"In contrast, the FDA has the benefit of the long view," Alito said. "Its drug-approval determinations consider the interests of all potential users of a drug, including those who would suffer without new medical products if juries in all 50 states were free to contradict the FDA's expert determinations."

In a telephone conference with reporters, Ms. Levine said she first heard about the high court's decision from an Associated Press reporter Wednesday morning. "I collapsed into tears for quite a while and that turned into jumping up and down," she said.She said she is grateful her case will help other injured consumers by establishing a precedent, and that she is thankful for the money judgment awarded by the Vermont jury that she can now collect. "It is not my arm," she said, "but it is just going to help so much in terms of my economic stress level."